Electronic apparatuses, which are so embodied that they satisfy ignition protection type Ex-d, must a have pressure resistant housing or a pressure resistant housing part. In this way, it is prevented that an explosion possibly occurring in the interior of the housing or in a portion of the interior of the housing can penetrate outwardly or into another portion of the interior, e.g. to the electronics in an adjoining space. In the standard, minimum values for the length and size of slits or gaps etc. between portions of the interior are fixed so that the slits or gaps etc. are flameproof.
In order that housings having pressure resistant parts have a sufficient mechanical strength, they are embodied with thick walls; thus, they are heavy and expensive. Constructing an electronic apparatus according to the ignition protection type Ex-d consequently requires a pressure resistant and therefore heavy and expensive, closed housing, or a housing with a correspondingly dimensioned housing part.
Housings for industrial applications include, for example, housings for accommodating the electronics of a measuring device, e.g. a pressure, fill level or flow measuring device. Such housings have a removable lid, e.g. so that the electronics is accessible for adjusting or parametering the same at the measuring location.
To allow such a housing to be applied in the food industry, it must satisfy special requirements. In this connection, there are hygienic requirements, according to which all seals contacting food must be replaceable; the housings have to be completely cleanable and the housings must have as few undercuts, in which deposits can accumulate, as possible. Additionally, the sealing of the housing must be assured in the face of long term utilization. This must also be assured inspite of its being cleaned regularly, e.g. by means of high pressure steam cleaning used in the foods industry. The pharmaceutical industry has comparable requirements. Moreover, housings for industrial applications must be corrosion resistant and chemically resistant.
Housings are commercially obtainable, in the case of which a lid is screwed onto a housing pot by means of a multiple start thread. The lid and housing pot are usually castings of aluminum or stainless steel. Such components are expensive. In order to assure a sufficient mechanical stability, these housings must have a wall thickness of at least four millimeters due to the screw thread. This wall thickness places a lower limit on the amount of material needed for the housing and therewith a lower limit for the weight of the housing.
Additionally, housings for industrial applications usually have a housing pot and a removable lid. Such arrangements are most often installed in the vicinity of the process or the medium to be measured and frequently are exposed to unfavorable process conditions reigning there, such as e.g. high environmental temperatures and heavy fouling or also rain, wind, dust, snow and ice in the case of an installation in the open.
It has been found that the screw connections usually used to connect a metal lid to a metal housing tend to seize. They can, after a certain length of time, no longer, or only with considerable effort, be loosened and, in individual cases, have to be broken or cut open.
A measure applied to this point in time to avoid the seizing of screw connections is the complete coating of the lid and housing, wherein, depending on the metal material of the lid or housing, a nickel coating, a lubricating lacquer or other powder coating, or an anodized or hard anodized aluminum coating is applied. A usual method is an immersion bath with a liquid providing the coating. The parts to be coated are completely immersed in the liquid. In cases where, for lids or housings of the past, no coating was desired, the concerned surface portion was covered by a lacquer or an adhered film before immersion in the coating bath. After the coating immersion bath, the covering must usually be removed, which is not only an additional working step but also requires the additional expense of a solvent.
Moreover, it should be noted that a complete coating of the lid and housing is complicated, expensive and not very environmentally friendly, because it requires a long residence time for the parts to be coated in the coating process.